The invention is in the field of well logging and relates particularly to a new kind of sonic logging which gives good results both in open and in cased boreholes and to utilizing such new kind of sonic logging to find subsurface parameters which are believed important in the search for and exploitation of hydrocarbons and other valuable underground resources.
Sonic well logs are typically derived from a tool suspended in a mud-filled borehole by a cable and taking sonic measurements every few inches (and for some tools--several times per inch) as it is slowly drawn up. Typically a sonic signal is transmitted from one longitudinal end of the tool and received at the other. The sonic signal from the transmitter enters the formation adjacent the borehole, and the arrival times and perhaps other characteristics of the receiver responses are used to find formation parameters such as the time required for a compressional soundwave to traverse one foot of formation. Typically the sonic speeds in the tool and in the drilling mud are less than in the formation and, accordingly, in uncased boreholes the first arrivals of sound energy at the receivers tend to correspond to sound-travel paths in the formation near the borehole wall. Sonic velocities in common formation lithologies tend to range from about 6,000 to about 23,000 feet per second but, to avoid small decimal fractions, the reciprocal of the velocity--slowness D--is typically recorded in microsec/ft over a typical range from about 44 microsec/ft for zero porosity dolomite to about 190 microsec/ft for water. A typical sonic log can be recorded on a linear scale of slowness versus depth in the borehole, and is typically accompanied by an integrated-travel-time log in which each division indicates an increase of one microsecond of the total travel time period. The integrated-traveltime log allows finding the travel time between any two depth levels by simply counting the divisions therebetween, and is useful for seismic purposes. Sonic logs are typically used as direct indications of subsurface properties or--in combination with other logs or other knowledge of the subsurface properties--to find subsurface porosity and other parameters which cannot be measured directly. In addition, it has been proposed to explore or exploit other properties of sonic signals in logging or other sound exploration of subsurface formations, and discussions related thereto can be found, as a nonlimiting example, in Aron, J., Murray, J. and Seeman, B.: "Formation Compressional and Shear Interval-Transit-Time Logging by means of Long Spacings and Digital Techniques", presented at the 53rd Annual Fall Technical Conference and Exhibition of the SPE, Oct. 1, 1978, Houston, Tex.; Stoffa, Paul L. et al: "Direct Mapping of Seismic Data to the Domain of Intercept Time and Ray Parameter: A Plane Wave Decomposition", paper presented orally at the 49th Annual Meeting of the Society of Exploration Geophysicists, November 1979, New Orleans, La.; Kokesh, F. P. and Blizard, R. B.: "Geometric Factors in Sonic Logging", Geophysics, Vol. 24, No. 1 February, 1959; Kokesh, F. P., Schwartz, R. J., Wall, W. B. and Morris, R. L.: "A New Approach to Sonic Logging and Other Acoustic Measurements", Jour. Pet. Tech., Vol. 17, No. 3 March, 1965; Hicks, W. G. and Berry, J. E.: "Application of Continuous Velocity Logs to Determination of Fluid Saturation of Reservoir Rocks", Geophysics, Vol. 21, No 3 July, 1956; Wyllie, M. R. J., Gregory, A. R. and Gardner, G. H. F.: "Elastic Wave Velocities in Heterogeneous and Porous Media", Geophysics, Vol. 21, No. 1 January, 1956; Wyllie, M. R. J., Gregory, A. R. and Gardner, G. H. F.: "An Experimental Investigation of Factors Affecting Elastic Wave Velocities in Porous Media", Geophysics, Vol. 23, No. 3 July, 1958; Tixier, M. P., Alger, R. P. and Doh, C. A.: "Sonic Logging", Jour. Pet. Tech., Vol. 11, No. 5 May, 1959; Tixier, M. P., Alger, R. P. and Tanguy, D. R.: "New Developments in Induction and Sonic Logging", Jour. Pet. Tech., Vol. 12, No. 5 May, 1960; Morris, R. L., Grine, D. R. and Arkfeld, T. E.: "Using Compressional and Shear Acoustic Amplitudes for the Location of Fractures", Jour. Pet. Tech., Vol. 16, No. 6 June, 1964; Pickett, G. R.: "Acoustic Character Logs and their Applications in Formation Evaluations", Jour. Pet. Tech., Vol. 15, No. 6 June, 1963; and Willis, M. E. and Toksoz N.: "Automatic P & S Velocity Determination From Full Waveform Digital Acoustic Logs", Massachusetts Institute of Technology, Cambridge, Mass., Jan. 16, 1981.
Prior art sonic well logging tends to give good and highly useful results in open (uncased) boreholes but not in cased boreholes. Indeed, no known prior art technique is known to give good sonic logs in cased boreholes despite the long standing need therefor, e.g., in cases where a sonic log run prior to casing turns out to be defective or not as good as later possible with earlier unavailable tools or techniques, or where no acoustic log was run at all prior to casing.
Accordingly, the invention provides a method and a system for acoustic logging which is successful both in open and in cased boreholes and which makes use of received sonic signal components which are not believed to have been successfully exploited for such purposes in the past and which give valuable clues of the nature of the subsurface formation. In accordance with the invention, new types of logs are produced, such as a slowness/time coherence log, and logs of other parameters associated therewith. The invented method and system are particularly useful with sonic logs derived from a tool in which the sonic receivers are close to each other--as a nonlimiting example, when the receivers are a foot or less from each other, and preferably less than half wavelength of the sonic signal (e.g., about six inches) from each other.
In a particular and nonlimiting embodiment, the invention starts with sonic logs which are records of the receipt, at successive borehole depth levels, of sonic signals by receivers which are spaced along the length of a borehole tool from each other and from at least one transmitter of sonic signals which is also carried by the same tool. These sonic logs are analyzed to find whether they include an arrival of sonic energy at time T and slowness D for all (T,D) combinations which have been found to be reasonable in practicing the invention. The measure of interest which is derived from these (T,D) combinations in accordance with the invention is called a coherence measure and is designated R.sup.2 (T,D). It corresponds to a measure sometimes called "semblance" in seismic work. It has been discovered that for each given depth level z in the borehole, the surface made up of the found measures of coherence for (T,D) combinations tends to have peaks corresponding to the different components of the sonic signal arriving at the tool receivers, e.g., the compressional, shear and Stoneley components. It has also been discovered that the arrival time and slowness associated with these peaks have unexpectedly significant relationship to the subsurface formation and that, accordingly, new logs associated with parameters of those peaks can be produced which give significant clues to the subsurface formation. As a nonlimiting example, one such new log is a log of the slowness associated with those peaks versus depth of the borehole. Another is a log of the arrival time associated with such peaks versus depth in the borehole or, for convenience of visual presentation, the difference between the arrival time of the coherent energy associated with a peak and the travel time thereof to the first receiver at the slowness associated with the peak. Yet another is a log of the received coherent energy associated with a peak or, for convenience of visual presentation, the attenuation of such energy in travelling from the transmitter to the receivers. Yet another can be the Poisson's ratio found from the slowness at each depth.
In a typical known prior art system the first sonic energy arrival at a receiver is generally assumed to be the compressional wave (P-wave) which has travelled from the transmitter to the receiver through the formation adjacent the borehole, the second arrival is generally assumed to be the shear wave and, if there is a subsequent arrival, it is generally assumed to be the Stoneley wave. In a cased borehole the P-wave is typically preceded by the arrival of the sonic wave travelling along the casing. However, it is not always true that the sonic energy arrives in the assumed order, and significant errors may be introduced if the incorrect initial assumption is made. In contrast, in the technique according to the invention the decision as to the significance of the received sonic energy is made on the basis of the entire waveform received by each of a number of receivers rather than on the basis of an initial assumption as to the significance of a portion of the waveform received at a particular receiver. In addition, a typical known prior art system relies on the difference in the time of arrival as between two receivers, and an error in any one of them may seriously affect the accuracy of the overall result. In contrast, in a system according to the invention reliance is placed on the synergistic combination of the energies received at many receivers, and an error in a minority of them is unlikely to seriously affect the accuracy of the system. Still in addition, a typical known prior art system relies on a pair of receivers spaced a couple of feet apart, with attendant limitations on vertical resolution, while in a system according to the invention the receivers are typically spaced from each other by a distance less than half the wavelength of the sonic signal from the transmitter (e.g., six inches apart), thereby significantly improving vertical resolution and reducing aliasing errors. Additional advantages of the invented process and system will become apparent from the detailed description below.